This invention relates to an alkali-free glass and, in particular, to an alkali-free glass for use as a light transparent glass substrate for a liquid crystal display and the like as well as a method of producing the same.
Conventionally, an alkali-free glass has been used as the light transparent glass substrate for the liquid crystal display and the like. The alkali-free glass for use in such a display is required to be free from bubbles such as blisters and seeds resulting in a display defect, in addition to various characteristics such as heat resistance and chemical resistance.
To meet such a demand, various kinds of alkali-free glasses have been proposed. Japanese Unexamined Patent Publication (JP-A) No. 63-74935 discloses an SiO2—Al2O3—B2O3—CaO—BaO as one of the alkali-free glasses.
In order to obtain a glass without bubbles, it is important to select a fining agent capable of generating a fining gas both in a relatively low-temperature range in which batch decomposition and degassing is started and in a relatively high-temperature range in which fining and homogenization of a glass are caused. Specifically, the fining agent serves to expel a gas generating during the batch decomposition and degassing of a glass material, and to enlarge very small bubbles remaining in the glass melt during fining and homogenization processes, whereby enlarged bubbles float up in the glass melt and leave the glass melt.
In the meanwhile, the alkali-free glass for use as a glass substrate for a liquid crystal display is high in viscosity of the glass melt so that a melting process is carried out at a high temperature in comparison with a glass containing an alkali component. In the alkali-free glass of the type, the batch decomposition and degassing generally occurs at 1200-1300° C. while the fining and the homogenization are performed at a high temperature of 1400° C. or more. Under the circumstances, the fining agent is required to produce the fining gas in a wide temperature range (on the order of 1200-1600° C.). Presently, As2O3 is widely used as the fining agent.
However, As2O3 is highly toxic and may possibly cause environmental pollution during a manufacturing process of the glass and during disposal of a waste glass. In this reason, the use of As2O3 is being limited.